Electrochemical process using a fluidized electrode

ABSTRACT

An extended surface area electrode for an electrochemical cell is provided by causing a bed of conducting particles to be expanded in the form of a fluidized bed by controlled upward flow of electrolyte through an electrode chamber containing current-carrying conducting means which makes contact with the fluidized bed.

This application is a continuation of application Ser. No. 288,387,filed Sept. 12, 1972, which is a continuation-in-part application Ser.No. 24,175, filed Mar. 31, 1970 which is a continuation-in-partapplication Ser. No. 639,803, filed May 19, 1967, all now abandoned.

This invention relates to extended surface area electrode arrangementsfor use in connection with electrochemical reactions.

As in most chemical reactions depending upon a surface effect, thereactivity of an electrochemical cell is dependent upon the active areaof the surface at which the reaction takes place, that is to say, thelarger the active surface area of an electrode per unit volume of anelectrochemical cell, the higher the current that can be supported by agiven volume of cell. Many proposals have been made for increasing theactive area of a solid type of electrode; thus it has been proposed, forexample, to provide an electrode in the form of an electronic conductorwith a surface which is activated by being made highly foraminous eitherby treatment of the conductor surface, such as by selective removal ofmetal according to a Raney process, or by use of particulate materialsmade adherent to the conductor surface; another proposal has been toprovide an electrode prepared by compaction of particulate materials toform a porous electronic conductor.

Although not the sole objective, it is one objective of the presentinvention to provide an electrode arrangement which is suitable forcells in which electrochemical reactions in the liquid state arerequired to be carried out in continuous flow. For such cells, the knownactivated solid type of electrode is very inefficient for reactionswhich occur at low rates, and, even with the maximum enhancement ofactive area that has been achieved with this type of electrode, the cellefficiency has not been as high as desirable. Moreover such electrodesare susceptible to temperature differences across the active surface.

Another known type of electrode, use of which might be thought toovercome some of these disadvantages, is one in which a mass ofelectronically conducting material and/or semi-conducting material inparticulate form is packed into a pervious electrode member--pieces,chunks and pellets have been proposed in this context, as well as smallparticles. In a cell making use of such an electrode, it is usual toprovide a permeable, or semi-permeable, membrane between this electrodeand an electrode of opposite polarity in order to avert contact betweenthe particulate material and the other electrode. While electrodes ofconsiderable surface area can be provided in this way, the difference inperformance from that of the solid type of electrode is not alwayssatisfactory since in practical configurations only a small fraction ofthe particles may be active due to the markedly non-uniform electrodepotential distribution.

Still further proposals have been made in the past to provide electrodearrangements in which particles of conducting or semi-conductingmaterial are caused to flow either constantly or intermittently throughan electrode chamber so that the particles may be changed or replenishedin the electrode during use. Thus it is known to provide a slurry orsludge type of electrode in which particles in constant contact witheach other are rendered mobile by means of a fluid, normallyelectrolyte, and the slurry or sludge is constantly or intermittentlycirculated through the electrode chamber.

In the case of the slurry type of electrode for a fuel cell, yet anotherproposal has been made to cause streams of electrolyte with finelydivided catalytic particles suspended therein to be circulated in thecell in such a way that the particles loaded with fuel or oxidisingagent are brought into contact with an electrode surface in theelectrode compartment.

Slurry and sludge type electrodes suffer from the disadvantage that bothparticles and electrolyte have to be circulated and therefore, in commonwith the circulating particle type, pose pumping problems, particularlyas regards the particles, as well as difficulties associated withelectrode potential distributions.

The present invention provides a means for enabling an electrochemicalcell to be operated with either or each electrode behaving as animproved extended area electrode.

Thus in accordance with an aspect of the invention, a method ofproviding an electrode for an electrochemical cell comprises providing amass of discrete particulate material in an electrode chamber containingcurrent-carrying means, said material being at least one member of thegroup consisting of (a) conducting material and (b) non-conductingmaterial coated with conducting material, and arranging for a liquidelectrolyte to flow through said chamber under control upwardly throughsaid mass so as to cause the particles to be levitated and thereby toform a fluidised bed of particles within the chamber with saidcurrent-carrying means in contact therewith, whereby said fluidised bedforms an extended area electrode for said cell.

According to another aspect of the invention, in the process ofoperating an electrochemical cell in which a reaction occurs involvingpassage of ions through an electrolyte between at least one anode and atleast one cathode, an improvement comprises:

(a) providing in the cell for one said electrode a vertically elongatedcolumnar zone containing current-carrying means,

(b) providing in said zone a mass of discrete particulate material, saidmaterial being at least one member from the group consisting of (1)conducting material and (2) non-conducting material coated withconducting material,

(c) forming said mass of discrete particulate material into a fluidisedbed of particles within said zone and in contact with saidcurrent-carrying means by controlled substantially uniformly distributedupward flow of liquid electrolyte through said zone, said fluidised bedof particles providing an extended area of surface for the electrode,and

(d) providing a membrane which is at least ion semi-permeable betweensaid fluidised bed electrode and the other electrode with electrolytebetween said other electrode and said membrane.

An improved process of operating an electrochemical cell according tothis latter manner involves providing for both an anode and a cathode tocomprise fluidised bed electrodes.

In operation, in a fluidised bed electrode as proposed according to theinvention, the upward velocity of the liquid electrolyte, which isarranged to be substantially uniform on a horizontal plane through theelectrode chamber or zone is adjusted so that each particle is levitatedby a drag effect of the electrolyte with the result that the bed or massof particles expands upwardly while still retaining its form in that itsupper surface is still discernible and substantially in a horizontalplane. The volume occupied by the expanded bed of particles is thereforegreater than the volume of the bed or mass of particles when static.This volume can be adjusted over a range depending upon the size of thechamber or zone and upon the sizes of the particles. If the rate of flowof the liquid electrolyte is raised to too high a value then theparticles will be swept out of the electrode chamber or zone and thefluidised bed electrode will have been destroyed; care is thereforenecessary to ensure that this does not happen unless intentionally,possibly to remove the particles from the chamber or zone so that afresh mass or bed of particles can be introduced to reform theelectrode.

The fluidised bed electrode provides considerable improvement becausethe particles are in a state of fairly vigorous motion against eachother and/or against the current carrying means. If a potential isapplied between the current carrying means and the other electrode ofthe cell, then it can be arranged that the particles can be charged anddischarged in rapid succession. Also by adjustment of the flow it ispossible to adjust the electrode potential of the electrode to optimumperformance. Such measure of control is not possible with the trappedparticle type of electrode, nor with the circulating slurry or sludgetype of electrode.

For certain reactions, the particles forming the bed may be such as toparticipate in the electrochemical reaction to be performed by the celland the size and/or size distribution of the particles may be chosen togive optimum results. The particles may be wholly of conductingmaterial, possibly of composite conducting materials or each or some ofthe particles may comprise a core, for example of glass or of a plasticsmaterial, coated partially or completely with conducting material, ormaterials. The particles of an electrode operating in accordance withthe invention will normally be fluidised by passage therethrough ofliquid electrolyte alone but the fluidisation may be effected by a fluidor fluids in addition to the electrolyte, for example one or morereactants.

Preferably the electrode arrangement is adapted to operate inconjunction with one or more current-conducting members extendingthereinto but it may be found, for particular uses, that thecurrent-conducting member(s) may form a boundary surface, or boundarysurfaces, or part of a boundary surface, of the electrode.

In accordance with another aspect of the invention an electrodearrangement for an electrochemical cell comprises at least one electrodechamber having a porous base, said chamber having a space beneath saidporous base and containing a layer of discrete particulate solids abovesaid base, said solids being at least one member of the group consistingof (a) conducting particles and (b) non-conducting particles coated withconducting material, means for introducing liquid electrolyte to thespace beneath said porous base and causing said electrolyte to flowthrough said base, the base being arranged to cause electrolyte flowthrough said layer of particles to be substantially uniform, means tocontrol said flow so as to cause the particles to be levitated, suchthat the layer is expanded to form and maintain a fluidised bed of saidparticles within said chamber, and at least one current-carrying memberextending into said chamber to contact said fluidised bed, whereby saidfluidised bed forms an extended area electrode for the cell. Thus theparticles as discrete solids will each be capable of bearing anelectrical charge and they are fluidised by flow of the electrolyteincluding the reactant, or one or more of the reactants if, for example,there is a multiplicity of reactants.

The charge-transferring particles may be of any suitable material(s),such as of metals or alloys, or of mixtures of metals and/or alloys orof non-metals coated with metals and/or alloys. They will normally be inthe form of powders and if the powder is comprised of particles ofsubstantially spherical shape, control of fluidisation can be moreeffective.

The invention is particularly applicable to electrodes for use with anelectrochemical cell comprising a membrane separating an anolyte from acatholyte; if the membrane forms a barrier that is of closedcross-section, the particles of the electrode may be positioned withinthe zone bounded by the membrane and the fluidisation of the particlesmay then be effected by flow of the catholyte, or the anolyte, throughthis zone. Alternatively, the boundary walls of the fluidised electrodemay include part of the cell containing-wall and/or the currentconducting members and may, for certain purposes, be used without amembrane.

It should be noted that an electrode according to the invention isdistinguished completely from one in which, as has already beenproposed, solid metal or semi-conducting particles form a sludge orslurry which have to be passed through the cell with a liquid reactant.It has been found that a fluidised bed electrode has considerable andunexpected advantage over the circulating particle type of electrode inwhich the particles are circulated past a current-carrying member.

In order that the invention may be better understood, one form ofelectrochemical cell embodying the invention will now be described withreference to the accompanying diagrammatic drawings of which:

FIG. 1 shows a form of electrochemical cell in which the cathode is offluidised bed form and,

FIG. 2 illustrates a cell having both anode and cathode of fluidised bedform.

Referring now to FIG. 1, the cell comprises a catholyte chamber formedby two flanged tubes 1 and 2 joined end to end with a porous plate 3between them. The tubes and flanges are of a porous plastics materialknown under the trade name "VYON" and manufactured by Porous PlasticsLimited and the tubes are 1 in. diam., with a wall thickness of 0.1 in.The pore size may be limited by availability of material to about 50microns, but other materials of different pore size may of course bechosen to suit any particular circumstances. Preferably the pore size isnot greater than the size of particles of the powder referred to belowwhich is introduced into the assembly. The plate 3 is of "VYON" sheet,1/16th in. thick and having about the same average pore size.

A cylindrical jacket 4 of lead is arranged to line a 2 in. diam. glasscylindrical envelope 5 and the envelope 5, together with the inner tubeassembly 1, 2 has end plates 6, 7 forming liquid-tight joints with boththe envelope and the inner tube assembly.

The end plate 6 carries a terminal 8 connected to the lead liner 4,which forms an anode for the cell, and a copper rod conductor 9 of 1/8thin. diam. passes both the end plate 6 and the plate 3, terminating nearthe bottom of the cell. An inlet tube 10 for the catholyte is providedin the end plate 7. Tubes 12 and 13 are provided in the end plates 6 and7 respectively to cater for flow of anolyte through the annulus formedbetween the envelope 5 and the inner tube assembly. The flow of anolytemay be in contra-flow to that of catholyte. The tube 1 serves as anion-permeable membrane.

A probe 14 from a reference electrode passes through the end plate 7 andis located close to the conductor 9 below the porous plate 3.

In accordance with the invention a quantity of powder, for examplecopper powder of size range 63 to 150 microns, preferably of a narrowrange, such as 63 to 75 microns, 75 to 90 microns or 125 to 150 microns,is contained within the inner tube assembly 1,2 so as, when static, torest on the porous plate 3, and this powder is caused to provide afluidised bed by flow of catholyte through the porous plate 3. Thedepiction of the fluidised bed of particles in FIG. 1, althoughdiagrammatic, illustrates the form of such a bed with its top surfacestill discernible in spite of the expansion of the static mass ofparticles.

Alternatively, the powder may comprise copper plated or metallized glassor polystyrene or other suitable plastics spheres and the sizes of thesemay be greater for similar flow because their densities are lower. Glassor plastics spheres may be produced of very uniform size and are readilyavailable over a large range of sizes, while solid metal and plated, ormetallized, glass particles of any of these size ranges are probablysuitable, although larger sizes may of course be used if desirable andpractical, it may be necessary in the case of particles with covers ofplastics material to select particles of larger sizes or to make certainthat the plastics materials of which they are composed is of higherenough density and/or that there is sufficient thickness of metalpresent as coating. This is to avoid working with opposite particleswhich have overall densities so close to the density of the fluidisingliquid that the movement of the particles when in the form of thefluidised bed is sluggish and not vigorous enough. Sluggish movementwill affect the performance of the electrodes.

For a given weight of particles in the bed it will be evident that theheight of the bed for a given geometry is dependent upon the flow rateof the electrolyte and on the size of the particles. Both particle sizeand bed height may require optimisation for any particular size of celland of the electrolyte composition.

The principle of operation of such a cell is not yet fully understood,but it has been found, for example, that the fluidised bed electrode ofthe dimensions given will easily sustain a current of 1 amp./cm.³ of thebed. It is possible that the particles of powder in the fluidised bedare charged as they come into contact with the central conductor 9 whichis maintained at a suitable potential difference relative to a referenceelectrode and that these charged particles then are discharged whencontacted by the reactant ions or molecules in the catholyte, or bycontact with other particles, the effect of the fine division of theelectrode material being to extend the active area of the cathode.

It has been noted, from observations of the performance of the cellillustrated in FIG. 1 for the cathodic reduction of m-nitrobenzenesulphonic acid to metanilic acid at room temperature, that for the samedegree of expansion of the beds, solid copper spheres in the two ranges75 to 90 microns and 125 to 150 microns sustain approximately the samepotential-current characteristics, that is between 9 and 10 amps. at apotential relative to a standard calomel electrode of about 0.70 volt.For the same bed height and potential, but using copper platedpolystyrene spheres in the range of 355 to 420 microns, the currentsustained is about 52 amps.

In terms of current densities, that is milli-amps per sq.cm., the solidcopper spheres in the two ranges mentioned immediately above, yield afigure of about 550-590 per unit area of the outside of the bed--that isabout 20 sq.cm. for the 1 inch height; for the copper coated polystyrenespheres of 355 to 420 microns the equivalent figure is over 3000.

Another type of cell is shown diagrammatically in FIG. 2, both the anodeand the cathode of the cell being comprised of fluidised beds ofparticles. Here the anode 20 is arranged within a cylindrical diaphragm21 of the porous "VYON" material, the internal diameter being about 3/4in. and the wall thickness 0.1 in. Surrounding the diaphragm is acathode 22 of annular form contained within the glass cylinder 23. Theanode particles, for example platinum powder, will be formed into afluidised bed of a desired height by electrolyte entering through theporous base 24; a typical position of the top of the anode fluidised bedis indicated at 25. Similarly a typical position of the top of theannular cathode fluidised bed is indicated at 26; this latter may be ofcopper particles, or indeed of any particles such as described above.The material of the particles for anode and cathode will, however,probably depend upon the reaction that is required to be carried out inthe cell.

The anode current feed is shown at 27 and terminates at a cylindricalgauze 28; the cathode current feed 29 terminates at a concentriccylindrical gauze 30. These gauzes are, as near as is practicable,concentric with the diaphragm 21.

This symmetrical arrangement appears to be advantageous so as to ensurethat the current density in the cell is reasonably uniform. However, ifthe current distribution in the cell can be controlled to ensure thatexcessively high local currents do not build up in parts of theelectrodes, then the need for symmetry of the electrodes is not sogreat.

The invention is applicable to cells for other types of reaction thanthose described above involving straightforward liquid electrolytes.Thus it is applicable to cells for use with reactants introduced ingaseous form together with liquid electrolyte. Also it is applicable incases where the reaction requires the electrolyte to be non-aqueous. Insome cases an ion semi-permeable membrane may be preferred to separatethe anode and cathode chambers.

An electrode operation according to the invention may be used for anyreactor provided that at least some part of the reaction required totake place is by electrochemical oxidation and/or reduction, or involvesa reaction wherein there may be a switch from reduction to oxidation orvice versa at the electrode or any reaction which in part involves anelectron transfer. Materials used in or composing the electrodefluidised bed may or may not be consumed during the reaction. It will beseen that arrangements can be made for replacement of some or all of theparticles in the bed without dismantling the cell and, if necessary,without destroying the form of the bed.

The invention has application to use in fuel cells wherein electriccurrent may be drawn from a cell as a result of the electrochemicalreaction in the cell, e.g. consumption of the particle material.

Although in the particular reactor that has been described above theelectrode material remains in situ, in certain reactions it may benecessary or desirable to circulate the fluidised bed, that is by takingoff some of the particles while replacing them with others, particularlywhere the particles enter into the reaction and are consumed orincreased in size. This enables the extracted reacting particles, forinstance, to be replenished by fresh particles which have been subjectedto a reconstituting process.

It is to be understood that the scope of the invention embraceselectrodes for use in batch type as well as in continuous orsemi-continuous reactors. In the latter types of reactor it is desirablethat there should be some form of automatic control and a means ofachieving such control may be made readily available in cells inaccordance with the invention by use of a probe such as the referenceprobe 14 in the cell described with reference to the accompanyingdrawing. Use of a reference potential taken off the reactor through suchprobe, enables a control of the conditions in the body of the electrodeto be effected, as by a straightforward feedback system through, forinstance, a potentiostat; by this means the electrode potential in thecell may be arranged to be kept constant or, if necessary, to followsome predetermined variable pattern. Such control systems will beevident to those skilled in the art.

In those forms of cell utilising a fluidised bed electrode in accordancewith the invention it will be observed that the flowing fluid can beused to control heat dissipation of the electrode and of any diaphragm,if present. By arranging for continuous or intermittent circulatory flowthrough a suitable means, a useful heat exchange process can beintroduced. However, in some forms of electrode, heat control will beinherent through the mere introduction of cool fluid for fluidisationpurposes.

It is to be understood that we do not rule out the possibility at thisstage of additional agitation of the electrode material being effectedby mechanical movement, for instance movement of the conductor extendinginto the electrode material or by some other mechanical means additionalto this conductor. For this purpose, the agitating means couldconceivably be a smooth rod which may have attachments that cause suchadditional agitation which may be effected in a liquid or even in thepresence of both liquid and gas.

It is to be understood that the form of the conductor to be adopted forthe cell may be any as found to be most suitable for the particular typeof cell. Thus rod conductors with metal gauze attachments and othersurface extending forms have been tried. It is clear, however, thatthese additions to the conductor should not interfere appreciably withthe flow conditions of the mobile solids in the electrode.

To summarise, the advantages over a conventional cell that can beachieved, either singly or in combination, by a cell operating with afluidised bed electrode in accordance with the invention, can be brieflystated as follows:

(a) Large surface area can be contained in a small volume of electrode.

(b) Simplicity of construction.

(c) Ease of addition or removal of heat.

(d) Ease of scale up in size.

(e) Ease of renewal or replacement of electrode material.

(f) Can provide continuous operation.

(g) Can easily be adapted to high pressure operation when gases areinvolved.

We claim:
 1. In the process of operating an electrochemical cell inwhich a reaction occurs involving passage of ions through an electrolytebetween an anode electrode and a cathode electrode, the improvementwhich comprises:(a) providing in the cell for one said electrode avertically elongated columnar zone containing current-carrying means,(b) providing in said zone a mass of discrete particulate material, saidmaterial being at least one member from the group consisting of (1)conducting material and (2) non-conducting material coated withconducting material, (c) forming said means of discrete particulatematerial into a fluidised bed of particles within said zone and incontact with said current-carrying means by controlled substantiallyuniformly distributed upward flow of liquid electrolyte through saidzone, said fluidised bed of particles providing an extended area ofsurface for the resulting fluidized bed electrode, and (d) providing amembrane which is at least ion semi-permeable between said fluidised bedelectrode and the other electrode with electrolyte between said otherelectrode and said membrane.
 2. A process of operating anelectrochemical cell as claimed in claim 1, which comprises providing anextended area fluidised bed electrode in the case of both anode andcathode.
 3. A method of providing an electrode for an electrochemicalcell which comprises providing a mass of discrete particles of a narrowsize range in an electrode chamber containing current-carrying means,said particles being selected from the group consisting of (a)conducting material, (b) non-conducting material coated with conductingmaterial and (c) combinations thereof, flowing a liquid electrolytethrough said chamber under control upwardly through said mass andcontinuing upwardly out of the upper surface of said mass so as to causethe particles to form a fluidised bed of particles within said chamberwith said current-carrying means in contact therewith, the volume ofsaid fluidised bed of particles being limited to less than that of theelectrolyte volume within said chamber by regulation of the rate of flowof the electrolyte to maintain said upper surface of said mass below theupper extent of said electrolyte within said chamber whereby saidfluidised bed of particles forms an extended area electrode for saidcell.
 4. A method of providing an electrode for an electrochemical cellwhich comprises providing a mass of discrete particles of a narrow sizerange in an electrode chamber containing current-carrying means, saidparticles being at least one member of the group consisting of (a)conducting material and (b) non-conducting material coated withconductive material, and flowing a liquid electrolyte through saidchamber under control upwardly through said mass so as to cause theparticles to form a fluidised bed of particles within the chamber withsaid current-carrying means in contact therewith, whereby said fluidisedbed forms an extended area electrode for said cell.
 5. The method ofclaim 4 wherein said particles are formed of copper metal.
 6. The methodof claim 5 wherein said particles have a size between about 63 and 150microns.
 7. The method of claim 4 wherein said electrode chamber has aporous base and said base is used to uniformly distribute the upwardflow of electrolyte for formation of said fluidised bed for particles.